Autonomous underwater vehicles (AUV's) are used in a variety of marine environments to explore beneath the water's surface. For example, AUV's may be used to perform marine surveys and to explore geological features of a sea bed as well as take various measurements of the underwater environment. AUV's are typically operated by a controller that may be installed on a ship. The ship controlling the AUV's mission may be referred to as a controller ship. The controller may be stationed at the surface of the water near to the area that the AUV is exploring. The controller may also assist with entry and extraction of the AUV into the marine environment. The AUV may be lowered into the water by the controller ship. When the AUV has completed its mission, it will typically return to the controller ship for recovery. The AUV may be given an acoustic homing signal in order for the AUV to navigate to the controller ship for recovery.
Recently, with advances in AUV technology and in particular, advances in fuel cells, underwater vehicles may travel for tens or even hundreds of kilometers before they must be re-fuelled. Hence, AUV's may be, at times, great distances from a controller ship. For example, an AUV may travel 50 kilometers from a controller ship. The controller ship may be immobile or it may be disadvantageous for the controller ship to move depending on the conditions of the marine environment. For example, when exploring in the arctic, it may be necessary to introduce an AUV into the water through a hole in an ice sheet. With thick ice covering, it may be impracticable for a ship to follow the AUV. Moreover, with a thick ice covering, the controller ship may drift with the ice flow, so that the AUV must navigate to a location for retrieval that is some distance from the point of launch. Hence, the homing system used to extract the AUV from the water must allow for transmission over reasonably large distances.
As is known in the art, low frequency acoustic signals suffer lower attenuation than high frequency acoustic signal, and so are favoured for use in long range homing systems. However, low frequency homing signals may coincide with the frequencies that are generated by other acoustic sources in the water (such as wave action, ships or wild-life). The AUV may become lost if it attempts to follow a false homing signal.
Techniques that overcome deficiencies in existing homing systems remain highly desirable.